HubSpot's lead owner is only assigned when a HubSpot user first messages them. Therefore, you'll need each of your teammates to add the HubSpot BCC address in order for the lead to be assigned to them.
Grabbing your BCC address from HubSpot
Go your Settings and on the Sidebar click "Sales" then "Log emails in CRM". From there, you'll see your HubSpot BCC address. Copy that for now.
Adding BCC into Interseller
After you've gotten your BCC address from HubSpot, copy and paste it into the BCC box in your email settings.